The present invention relates to a light diffusing film, and more particularly to a light diffusing film which, especially when used in various displays, lighting equipment, and display boards, can realize high luminance and high visibility, and a process for producing the same, a surface light source, and a display device.
In various displays, lighting equipment, or display boards, light diffusing films are used for uniformly spreading light from a light source to enhance visibility.
Conventional light diffusing films include: a light diffusing film using a film comprising a light diffusing agent dispersed in a film substrate of a light-transparent rein, such as polymethyl methacrylate resin or polycarbonate resin; a light diffusing film comprising a film substrate and concaves and convexes provided directly on the surface of the film substrate, the film substrate being formed of a light-transparent resin, such as polymethyl methacrylate resin or polycarbonate resin, or being formed of a light-transparent resin, such as polymethyl methacrylate resin or polycarbonate resin with a light diffusing agent dispersed therein; and a light diffusing film having a light diffusing layer formed by coating a film substrate with a composition comprising a light diffusing agent compounded and dispersed in a light-transparent resin.
As shown in FIG. 9, a light diffusing film 31 is generally provided between a light-guide material 33 having scattering dots 32 and a liquid crystal panel 34 and serves to diffuse light from a light source 35. A large number of light diffusing films have hitherto been proposed in the art.
In some cases, a prism sheet is disposed between the light source and the liquid crystal panel from the viewpoint of focusing light from the light source to enhance luminance at the front of the liquid crystal panel. In this case, the prism sheet is used in combination with the light diffusing film.
Up to now, however, the structure of the combination of the prism sheet with the light diffusing film has not been fully taken into consideration. Consequently, disadvantageously, the prism sheet could not have satisfactorily improved luminance.
An attempt to provide a light diffusing film compatible with the optical properties of the prism sheet for the purpose of solving the above problem has posed a problem that, in prior art methods, such light diffusing films capable of attaining the above purpose are difficult to produce and cannot be stably produced.
Further, when the use of a light diffusing film is contemplated, the function of concealing the scattering dots provided on the light-guide plate should be imparted to the light diffusing film.
Under these circumstances, the present invention has been made, and it is an object of the present invention to provide a light diffusing film which can be easily and stably produced and, in addition, possesses excellent light diffusing properties, and, especially when used in combination with a prism sheet, can improve the luminance of light from a light source, a process for producing the same, a surface light source, and a display device.
The above object of the present invention can be attained by a light diffusing film, comprising: a transparent substrate; and a light diffusing layer stacked on at least one side of the transparent substrate, the light diffusing layer being formed of an ionizing radiation-cured resin, the light diffusing layer having on its surface fine concaves and convexes, the light diffusing layer having a surface roughness of 1 to 10 xcexcm in terms of ten-point average roughness Rz and an average inclination xcex8a of 5 to 20 degrees.
According to a preferred embodiment of the present invention, the light diffusing layer is free from light-diffusing particles.
According to another preferred embodiment of the present invention, the light diffusing film has been constructed so as to have a haze of 85 to 95% and a total light transmittance of 80 to 90%.
According to a preferred embodiment of the present invention, the light diffusing film has been constructed so that, when the angle of light incident on the light diffusing film is 60 to 80 degrees to the normal of the light diffusing film, the peak of the angle of outgoing of light is 30 to 50 degrees to the normal of the light diffusing film.
According to the present invention, the light diffusing film may further comprises a matte layer on the transparent substrate on its side remote from the light diffusing layer.
According to the present invention, there is provided a process for producing a light diffusing film, comprising the steps of:
winding a transparent substrate on the circumference of a roll form having on its surface fine concaves and convexes while applying an ionizing radiation-curable resin in a fluid state onto the surface of the roll form and rotating the roll form while bringing the transparent substrate into contact with the ionizing radiation-curable resin;
during the rotation of the roll form, bringing the ionizing radiation-curable resin into intimate contact with the transparent substrate and curing the ionizing radiation-curable resin; and
separating the transparent substrate together with the cured resin from the roll form.
Further, the present invention includes a surface light source comprising:
a light-guide plate which permits light to enter therein from a side end thereof and to go out from an outgoing face orthogonal to an incident face;
a light source installed on at least one end of the light-guide plate; and
the above light diffusing film provided on the outgoing face of the light-guide plate.
The surface light source may further comprise a prism sheet provided on the light diffusing film.
The present invention also includes a display device comprising a display panel provided on the above surface light source.